Caused by rB3b6ee8cee708: Refactor: Move vertex group names to object data

The raw data is copied here correctly
[BKE_gpencil_stroke_weights_duplicate in
BKE_gpencil_stroke_duplicate] but the vertex groups names are missing.
Prior to above commit is was enough to have ED_object_add_duplicate
(this seemingly took care of duplicating object's defbase).
Now vertex groups names sit on the bGPdata rather than the Object,
and since the separate operation creates new bGPdata we have to
copy vertex groups names - and active index - over [via BKE_defgroup_copy_list].